At the start of the game, when approaching a dark void found in Ordon Village, a Twilight Messenger grabs Link by his neck and pulls him into the twilight. For the first part of the game, Link is stuck in his wolf form, and you can only become human again by collecting Tears of Light. From this point on, every time he enters the Twilight Realm, Link will turn into a wolf.
When he finds the Master Sword after the Lakebed Temple, the crystal drops out of Wolf Link's head. (Zant put it there when he injured Midna). Link can now use this to choose to be a wolf or human. Link's wolf form is a lot quicker than his human form. Other special abilities include the ability to attack with his fangs, dig holes to uncover buried items, and use his wolf senses to see invisible spirits and smell scents left by certain objects. Midna will always be on Link's back as a wolf. She will occasionally give him hints on how to progress through the game. She could also use an energy field to attack enemies and help Link jump longer distances.
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